Transfer of electrostatic images (TESI) from a photoconductor acting as the primary image receiver to a dielectric surface is well known in the art (cf. Electrophotography, R. M. Schaffert, pp. 167-177, Focal Press, 1975). In a typical charge transfer process, a photoconductive layer bearing a conventionally made charge image is positioned near a dielectric receiving layer and a voltage of suitable polarity is applied between conductive substrates on the sides of these layers facing away from each other. The positioning of the layers must be such that a dielectric breakdown of the air between the layers can occur when a reasonable maximum voltage (e.g., typically less than 2000 volts) is applied. The dielectric receiving layer is then removed from the photoconductor while maintaining a biasing voltage. At a critical point in the separation, discharge currents flow across the air gap so as to transfer at least some of the original image charge on the photoconductor in an imagewise fashion to the dielectric receiving layer. This transferred electrostatic image may be made visible by conventional toning techniques. Variations on this technique have been developed and are described in the art. However, the importance of the thickness and uniformity of the gap between the donor and receptor is a factor in them all.
To obtain good quality images it is desirable during the transfer step to maintain a precise air gap between the photoconductive and receiving layers. Air gap separations of the order of a few microns have generally been thought to be desirable. If the gap is too large, little or no charge will transfer; while if it is too small, there can be considerable transfer of charge in the background areas resulting in a mottled background. In addition, because the relationship between the voltage needed to cause dielectric breakdown in the air gap and the air gap spacing (the Paschen curve) is not constant, a uniform air gap spacing is desirable for high quality transfer images.
Processes known in the prior art for the transfer of electrostatic images (TESI) have found practical application in commercial electrophotographic or electrostatic printing only for low resolution images.
In electrophotography or electrostatic printing, the prior art techniques for accomplishing charge transfer from one surface to another involves either: (1) conduction of electric charges across an air gap, or (2) direct charge transfer if the air gap is eliminated. While the air breakdown charge transfer technique is simple, it does not provide high resolution (less than 80 line pairs per millimeter (lp/mm) can be achieved) or continuous tone gray scale reproduction. Finally, this method also requires the donor surface to sustain high surface potentials to insure air breakdown. The presently known techniques for direct charge transfer require very smooth surface, a transfer liquid interfacing the donor and receptor films, or very high pressures to eliminate the air gap. Even though high resolution of up to 150 lp/mm charge transfer has been claimed, these techniques are impractical and the charge transfer efficiency is generally low. Accordingly, there remains a need for a simple means of making high resolution charge transfer images with gray scale fidelity and high transfer efficiency.
One aspect of the invention is to provide an efficient charge donating photoconductive-insulative surface.
Another aspect of the invention is to efficiently transfer a high resolution latent electrostatic charge image from the charge donating photoconductive-insulative surface to the charge receptor medium while these surfaces are in virtual contact.
U.S. Pat. No. 2,825,814 teaches a method for maintaining spacing by placing between the surfaces of the photoconductive and receiving layers a small quantity of powdered resin or plastic which is obtained by grinding the material to a relatively uniform particle size. However, the dusted particles tend to adhere to both surfaces, the final image areas often contain blotches caused by the presence of the particles used to maintain the spacing, and the resin particles and thus the spacing are not uniform. These disadvantages result in poor transferred images upon toning.
U.S. Pat. No. 3,519,819 discloses maintaining a spacing by coating a thin layer of electrically insulating film forming polymeric binder containing particulate spacer particles. These particles are embedded in the polymer binder layer in such a manner that the amount by which these spacer particles protrude determines the air gap thickness. However, because the particle size distribution of the spacer particles is random and each particle is not deposited in the same orientation within the binder, the amount by which each particle protrudes about the substrate is not uniform. Thus a uniform air gap cannot be achieved readily.
U.S. Pat. No. 3,240,596 teaches the use of direct contact between the photoconductive layer and the dielectric receiving layer in an imaging process. The charge transfer is slow and inefficient with a large amount of bias or background charge being transferred. This causes mottling in the background and a generally poor image.
U.S. Pat. No. 4,263,359 teaches the use of microdots of a photopolymerized composition on the receptor layer to provide uniform spacing in the air gap between the dielectric receiving layer and the photoconductor layer. This technique improves the consistency of the spacing between the layers, but charge transfer must still be effected by breakdown in the air gap and with an attendant bias voltage applied. Charge transfer is also quite slow and inefficient.